3,436. Trading Maaser Ani for Other Produce

Hilchos Maaser 7:9

If someone has produce in a silo and gives a seah (a measure of volume) to a Levi and a seah to a needy person, he may not put aside eight seah from the silo and eat from it unless he knows for sure that the two seah of tithes that he gave still exist. [This is because we can’t take produce for tithes we already gave unless these tithes are still around.] If the Levi or the needy person already ate from them, the landowner may only put aside a volume commensurate with what remains from those two seah of tithes.

Hilchos Maaser 7:10

Let’s say that needy people exchange the gifts they received with the owner of the field. For example, they give him a seah of leket, shich’chah, peah or tithes for the needy, in exchange for which they receive a seah of grain from the granary. In such a case, the grain the landowner receives is exempt from tithes even if it became ready while in his possession. Tithes must be taken from the produce that the needy receive from the landowner even though it was given in exchange for gifts that are exempt.